A crystal device for surface mounting such as a crystal unit, a crystal oscillator, or a crystal filter is known as a frequency control element. For example, a crystal unit for surface mounting (hereinafter, referred to as a surface-mounted unit) is integrated into an oscillator circuit, to be built into various types of electronics as a source of frequency or as a time reference. In recent years, the miniaturization thereof has been further advanced, which has brought about their planar outlines which are 2.0×1.6 mm or less, for example.
FIG. 3 are diagrams for explanation of a related art surface-mounted unit, in which FIG. 3(a) is a cross-sectional view of the related art surface-mounted unit, and FIG. 3(b) is a plan view of the related art surface-mounted unit without a metal cover.
The related art surface-mounted unit is configured such that a crystal element 2 is housed in a ceramic case 1 in a rectangular form in plan view, whose cross section is formed to be concave. A metal cover 3 is jointed to an end face of the opening of the ceramic case 1 by seam welding, to hermetically encapsulate the crystal element 2. The ceramic case 1 is composed of a bottom wall 1a and a frame wall 1b, and a metal ring 4 is fixed to the upper surface of the frame wall 1b serving as an end face of the opening by, for example, silver solder (not shown).
In this case, the outline of the metal ring 4 is made smaller than the outline of the ceramic case 1, to prevent the metal ring 4 from protruding from the end face of the opening of the ceramic case 1. That is, the metal ring 4 is formed to have outside dimensions for estimated clearances (error tolerance) Then, the inner circumference of the metal ring 4 is made to basically correspond to the inner circumference of the frame wall 1b of the ceramic case 1, and the widths of the long side and the short side of the metal ring 4 are made identical to one another.
Then, external terminals 5 are provided to the four corners of the outer bottom face of the ceramic case 1 (bottom wall 1a) and crystal holding terminals 6 are provided to the both sides of one end of the inner bottom face. A pair of the external terminals 5 obliquely facing each other in the four corners of the outer bottom face is electrically connected to the crystal holding terminals 6 on the inner bottom face through a lamination plane and unillustrated through holes (electrode through holes). The other pair of the external terminals 5 obliquely facing each other is electrically connected to the metal ring 4 through electrode through holes and the like.
The crystal element 2 is formed as an AT-cut crystal element, and is formed into a rectangular form, which is long, for example, in a direction of the X axis among crystal axes (XY′Z′), and a thickness of the crystal element 2 is the Y′ axis and a width of the crystal element 2 is the Z′ axis. The AT-cut crystal element oscillates in a thickness-shear oscillation mode, and the direction of the X axis is a displacing direction of the thickness-shear oscillation. The crystal element 2 has excitation electrodes 7 on its both principal surfaces, and leading electrodes 8 are extended on the both sides of one end serving as outer circumferential portion thereof. The extended both sides of the one end of the crystal element from which the leading electrodes 8 are extended, are fixed to the crystal holding terminals 6 by an electrically conductive adhesive 10. Thereby, the extended both sides of the one end of the leading electrodes 8 are electrically and mechanically connected to the crystal holding terminals 6. The metal cover 3 is formed into a planar outline which is smaller than the planar outline of the metal ring 4 for estimated clearances as described above.
Seam welding energizes between the pair of roller electrodes while making a pair of roller electrodes (not shown) contact the one end side of a set of sides facing each other of the metal cover 3 to press those to rotate so as to travel to the other end side. Thereby, an Ni (nickel) film on the outer circumferential surface of the metal cover 3 is fused by Joule heat so as to be jointed to the metal ring 4. Then, after the set of the sides facing each other of the metal cover 3 are jointed to the metal ring 4 the other set of sides facing each other are jointed thereto in the same way.
In these cases, the facing surface of the metal cover 3 with respect to the metal ring 4 is basically jointed to the metal ring 4, and a width of the joint surface (facing surface) of the metal cover 3 with respect to a width of the metal ring 4 is a so-called sealing path. In this case, if a welding current serving as one of the welding conditions is made too low, unjointed portions are generated to deteriorate its airtightness. In contrast thereto, if a welding current is made too high, splashes are caused to make metal dust splash inside the ceramic case to adhere to the crystal element 2 for example, which deteriorates its oscillation property.
For this reason, welding conditions such as a welding current including a rotational speed of electrode rollers are strictly limited. However, in reality, if welding conditions, for example, a welding current is strictly limited within a narrow range, a welding current varies depending on a power fluctuation or the like, which cause the occurrence of airtight loss or splashes. Then, normally, a width of the metal ring 4 is broadened to broaden a width of the facing surface of the metal cover 3. Thereby, not only is airtightness ensured and the occurrence of splashes suppressed, but also welding conditions are relaxed by raising especially an upper limit of a welding current to ensure its airtightness and to improve its productivity.
{Citation List}
{Patent Literature}
{PTL 1} JP-A-2007-173976
{PTL 2} JP-A-2007-75857